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Common Symptoms of Bad Spark Plug – How To Replace Them?

Knowing the early warning signs of spark plugs will help to replace them before they fail completely, and thereby prevent engine damage and costly engine repairs.

Spark Plugs will ensure optimal combustion performance in your vehicle without causing any damage to the electrodes. However, a timely replacement of these spark plugs is required, if not, the engine’s performance, reliability, and longevity would be affected.

Driving your vehicle with bad spark plugs, the engine might experience various problems. Bad or faulty spark plugs can happen for various reasons that impact the combustion. Here, in this article, we are going to discuss the symptoms of bad spark plugs and the replacement of spark plugs.

What are Spark Plugs and What Do They Do?

Here’s a clear understanding of spark plugs and their vital role in Internal Combustion Engines:

Spark plugs are vital components that play a pivotal role in the functionality of internal combustion engines, commonly found in motorcycles, cars, and lawnmowers. Despite their compact size, their significance is monumental. These electrical devices are threaded into the engine’s cylinder head, featuring a threaded metal shell, a central electrode, and an insulator. The central electrode protrudes and extends into the combustion chamber, while the metal shell is connected to the engine’s ground. The design and materials of a spark plug are crucial for its performance, as it should endure the high temperatures and pressures within the engine.

What Do Spark Plugs Do?

Essential Role of Spark Plugs in Engine Ignition:

Spark plugs serve a crucial function in the internal combustion process by igniting the air-fuel mixture within an engine’s cylinders. As the engine operates, it draws a combination of air and fuel into each cylinder. Before this mixture can power the engine, it requires ignition, and thus the spark plug plays a pivotal role.

The spark plug facilitates the flow of electrical current from the ignition system, generating a spark at the tip of the central electrode. This spark, in turn, ignites the compressed air-fuel mixture that initiates a controlled explosion. This explosion generates the required force to drive the engine’s pistons, propelling the vehicle forward. Essentially, spark plugs act as tiny yet powerful ignition sources, ensuring smooth and efficient engine operation.

What are the Failing Signs of your Spark Plugs?

Clean spark plugs of the engine aid in the optimal performance of the vehicle. If the spark plugs become dirty, bad, or fouled, then it affects the running functionality of the engine. Driving with fouled or bad spark plugs can cause a host of problems for the engine.

Identifying the faulty or bad spark plug symptoms is crucial to maintaining your vehicle’s performance. Here are some of the common signs and symptoms of a bad spark plug that require prompt immediate action and replacement:

1. Rough Idling

  • Sign – Noticeable changes in the vehicle’s idle – resulting in shaky/vibrating, making unpleasant noises, and causing rough idling.
  • Inference If you hear any rattles, metallic pings, or sputtering sounds rather than a smooth, even sound after you start and run the engine (or) sitting at a stoplight or stop sign. Then it indicates misfiring spark plugs, as they won’t burn fuel completely, and resulting in fluctuating RPMs & a louder noise.

2. Poor Acceleration

  • Sign – Delayed or sluggish acceleration that doesn’t pick up speed, even pressing the gas pedal.
  • Inference – Faulty spark plugs fail to effectively ignite the air-fuel mixture. They are unable to generate a sufficient spark, allowing them not to fully utilize the fuel injection and thereby irregular ignition. Spark plugs might also affect acceleration from startup (hesitate to keep up and accelerate).

3. Decreased Fuel Efficiency

  • Sign – Increased trips to the gas station due to reduced fuel efficiency.
  • Inference Faulty spark plugs lead to incomplete engine combustion, and make this combustion chamber receive debris. The worn-out plugs reduce the fuel economy by almost 30% (bad mileage – drops by 3 miles per gallon, and wastage of fuel).

4. Engine Misfires

  • Sign – Sudden jerking or jolting sensation, especially during acceleration or under load.
  • Inference – Worn-out spark plugs lead to incomplete combustion and not having enough force. Spark plugs fail to generate sparks consistently, resulting in engine misfires. Thus, it leads to intermittent engine sounds, carbon deposits on the spark, performance issues, and other mechanical issues.

5. Hard Starting and Difficulty Turning Over

  • Sign – The engine struggles to turn over or takes a long time to start (difficulty starting), especially in cold weather.
  • Inference – Indicates potential spark plug issues affecting the starting process. A properly functioning spark plug will easily start the engine by igniting a cold fuel mixture, while weary spark plugs won’t produce enough spark to ignite the fuel and start the engine or turn over the vehicle.

6. Poor Engine Performance

  • Sign – Overall reduction in engine performance, including power and responsiveness.
  • Inference The vehicle becomes sluggish and less capable of handling various driving conditions. Here the reduced spark capacity results in numerous issues, and one among them is improper ignition of the fuel mixture. The vehicle will run in a lean condition with the lowered fuel injection.

7. Increased Emissions

  • Sign – Dark, sooty smoke from the exhaust pipe or failing emissions tests.
  • Inference – Malfunctioning spark plugs contribute to incomplete combustion and higher emissions. If you notice any enhanced emissions in the engine, then it’s time to check and address the issues with your spark plugs.

8. Engine Knocking or Pinging

  • Sign – Unusual noises (pinging sounds), particularly under load or when climbing hills.
  • Inference – Spark plug issues leading to irregular combustion and engine knocking. If you’re driving a car with a 4, 6, or 8 cylinder engine, a really loud noise from the engine is due to malfunctioning spark plugs.

9. Poor Throttle Response

  • Sign – Delay in the engine’s response upon pressing the gas pedal, especially during overtaking or merging onto highways.
  • Inference – Spark plug problems affecting throttle response. This delay in the engine’s response is particularly noticeable while trying to overtake other vehicles or merge onto highways.

10. Check Engine Light

  • Sign – Illumination of the check engine light on the dashboard.
  • Inference – With insufficient combustion and constant misfire, the vehicle indicates a problem detected by the onboard diagnostics system (OBD scanner) and turns on the check engine light. Thus, it prompts the visual inspection of the spark plugs.

11. Lean Condition

  • Sign – Spark plugs trick the oxygen sensors resulting in a lean condition
  • Inference – Since the spark plugs ignite to burn the fuel mixture. If they go bad, spark plugs will trick the oxygen sensors, resulting in a lean condition. The fuel is dumped into the oxygen sensor, creating a lean condition. However, sensors causing lean fuel mixture take time to send the information.

Verdict: While these are indicative spark plug failure symptoms, they can also overlap with other engine or ignition system problems. If you suspect spark plug trouble, then it is advisable to consult your user manual and consider seeking professional automotive diagnostics (if needed). However, performing timely repairs ensures efficient and safe vehicle operation.

What Causes Spark Plugs to Go Bad?

Understanding the factors that contribute to the deterioration of spark plugs is crucial for preserving your vehicle’s performance. Here are the main reasons why spark plugs may degrade or fail:

1. Normal Wear and Tear

Over time, the constant exposure to extreme heat and pressure within the engine naturally wears out spark plugs. The degradation of electrodes and insulators diminishes the ability of the spark plug to generate a strong spark.

2. Carbon Buildup

Accumulation of carbon deposits over time on the spark plug electrodes and insulator can hinder the capability of the spark to jump the electrode gap and ignite the air-fuel mixture effectively. This buildup can result in misfires and poor combustion.

3. Oil Fouling

If you find any oil leak (or) issue with the piston rings or valve seals can allow oil to seep into the combustion chamber and foul the spark plugs. Oil-fouled spark plugs appear black and oily, compromising their functionality.

4. Fuel Additives

Fuel additives (either using excessive amounts or low quality) result in spark plug fouling. These additives can leave deposits on the plug, affecting its performance.

5. Incorrect Spark Plug Gap

Using wrongly/improperly gap-sized spark plugs for your vehicle’s engine causes misfires and poor combustion. It’s vital to adhere to the manufacturer-recommended spark plug gap for your specific vehicle (make and model).

6. Overheating

An engine running too hot can lead to spark plug overheating, causing electrode erosion and eventual failure. Cooling system malfunctions or engine running under excessive loads can contribute to overheating.

7. Pre-Ignition or Detonation

Premature ignition of the air-fuel mixture caused due to excessive heat or pressure can damage spark plugs. Thus, it creates abnormal combustion forces and high temperatures.

8. Contaminants in the Air-Fuel Mixture

Contaminated or dirty fuel, along with malfunctioning air filters, can allow impurities to enter the combustion chamber. These contaminants can easily impact the longevity and overall performance of the spark plugs.  

9. Excessive Idling

Since the combustion process is less efficient at low engine speeds, the extended periods of idling result in the spark plug fouling. However, driving regularly at higher speeds will surely help you to get rid of this issue.

10. Short Trips

Keep in mind that frequent short trips won’t allow the engine to reach the desired operating temperature, and thereby contribute to the fouling and carbon buildup of the spark plugs. However, the best solution is to go for longer drives that certainly burn off these deposits.

11. Old Age

Infrequent or rarely driven vehicles will degrade spark plugs over time due to exposure to temperature fluctuations, moisture, and other environmental factors. It’s advisable to replace spark plugs according to the manufacturer’s recommended maintenance schedule, irrespective of mileage.

Verdict: To ensure optimal engine performance and fuel efficiency, it is crucial to regularly monitor your spark plugs and replace them as necessary. Following recommended maintenance schedules, including timely spark plug replacements, can prevent many of these issues and ensure a smoothly operating engine.

When To Replace Spark Plugs?

As a user, you should know when to replace spark plugs to maintain the optimal performance of your vehicle. Here are guidelines that have to be followed to make informed decisions:

1. Mileage-Based Replacement

Replace spark plugs every 30,000 to 100,000 miles, depending on the type (copper, platinum, or iridium) and manufacturer recommendations. Ensure that the plug should be replaced timely for its effective functioning.

2. Consult the Owner’s Manual

Refer to the owner’s manual of your vehicle to know more about its precise maintenance intervals, tailored to your vehicle’s specifications. It provides valuable insights into when spark plug replacement is due.

3. Performance Signs

If you observe any signs of diminished performance, including rough idling, reduced fuel efficiency, engine misfires, or difficulty starting, then it’s time to replace spark plugs. Also, it is essential to address these issues promptly to prevent further engine damage.

4. Coordinated Tune-Up

Concurrently consider replacing the spark plugs along with other major tune-up components, such as air filters and ignition coils. This ensures comprehensive maintenance and enhances overall engine efficiency.

5. Professional Guidance

If unsure of its replacement schedule, seek advice from a mechanic for a professional assessment and recommendations. Follow a regular maintenance schedule for timely replacements, and thereby better functionality of spark plugs to enhance the reliability and longevity of the ignitions system.

Verdict: By combining all these factors, you can make informed decisions about when to replace spark plugs, and this proactive approach promotes fuel efficiency and overall engine health.

How To Replace Spark Plugs?

However careful steps and the right tools are required to replace the spark plugs efficiently. Here’s a concise guide.

  • Before starting the process, you need to gather all the necessary tools and materials, such as a spark plug socket, correct replacement spark plugs, a ratchet, and a gap tool.
  • Ensure that your vehicle’s engine is cooled down completely before starting this replacement process.
  • Disconnect the battery to get rid of any electrical accidents.
  • Locate and access the spark plugs, which are commonly found on the cylinder head.
  • Use a spark plug socket to loosen and remove the old spark plug. Ensure to remove the spark plug wire/boat slowly and carefully one at a time.
  • Then inspect the gap of the new spark plug with a gap tool and adjust them as needed to match the manufacturer’s specifications.
  • Hand-thread the new spark plug carefully into the cylinder head and then use the socket and ratchet to tighten them properly (don’t over-tighten them).
  • After successful tightening, firmly reattach the plug wire or boot onto the new spark plug.
  • Repeat the same procedure for each spark plug one at a time to prevent wire mix-ups.
  • Reconnect the battery and start the engine to check its proper installation and its potential misfires or issues (if any).
  • Finally, start the engine to check/verify if it runs smoothly or not. If everything runs smoothly, then you have succeeded in replacing the spark plugs.

Verdict: Follow this step-by-step procedure for the successful replacement of the spark plugs. In case, you are not sure about its replacement, then simply consult a professional mechanic to perform this task.

How Much Does this Spark Plug Replacement Cost?

The cost of spark plug replacement varies based on factors such as plug type, vehicle’s make and model, and the choice between DIY replacement or professional service. Typically, a set of spark plugs for a standard vehicle will range from $16 to $100 (or more), with premium iridium or platinum plugs being pricier. If opting for a mechanic, labor costs may add $50 to $200 or more (to the overall expenses). However, DIY replacement is feasible for those with basic automotive skills, potentially saving on labor expenses.

How To Determine if the Spark Plug Has Gone Bad/Wrong?

Follow these steps carefully to determine if a spark plug has gone bad/malfunctioning.

  • Visual Inspection – Check for the signs of any physical damage like cracks or deposits on the spark plug’s insulator.
  • Electrode Color Check – Inspect the color of the spark plug’s electrodes. If it is in a light tan or gray, then it is ideal and perfect. In the case of dark or black deposits, it indicates some potential issues.
  • Gap Measurement – Use a gap tool to measure the spark plug gap, and then compare the measurement with the manufacturer’s specifications.
  • Fouling Inspection – Look for signs of oil or fuel fouling on the spark plug, indicating possible engine problems.
  • Visual Comparison – Conduct a visual comparison of all spark plugs for inconsistencies, which might reveal problems with a specific cylinder.
  • Testing Tools – Utilize a scan tool or an ignition tester to identify misfires or irregular firing patterns. This indicates the bad spark plugs.
  • Listen for Symptoms – Pay attention to engine misfires, rough idling, or poor acceleration, as these can be often symptoms that indicate faulty spark plugs.
  • Monitor the Fuel Efficiency – A constant monitoring of fuel efficiency will let you know of any significant drop in miles per gallon. This indicates that there are some issues with the spark plug.
  • Scheduled Replacement – If you want to avoid problems proactively, then consult your vehicle’s maintenance schedule or replace spark plugs based on the manufacturer’s recommended intervals.


There you go! We’ve discussed the common signs and symptoms of worn-out/bad spark plugs and provided a step-by-step guide for their replacement. Despite their small & compact size, these spark plugs play a crucial role in maintaining the engine’s smooth and efficient operation. By staying alert to the warning signs and adhering to replacement guidelines, you can avoid the hassle of sluggish performance, diminished fuel efficiency, and unexpected breakdowns.

Whether you’re a DIY enthusiast or prefer professional assistance, maintaining your spark plugs properly will ensure optimal performance from the ride. We hope you get a detailed insight into the symptoms of bad spark plugs. Also don’t hesitate to share your experience and queries regarding signs of bad spark plugs in the below comments section. We will respond promptly to resolve your questions and help you replace the faulty or bad spark plugs. 

So, go ahead, ignite the engine, and drive with confidence.

Symptoms of Bad Spark Plug – FAQs

1. How long do these spark plugs last?

Ans: The lifespan of a spark plug depends on its type, ranging from 10,000 to 100,000 miles. A standard copper spark plug typically lasts between 10,000 to 20,000 miles, while silver plugs in older vehicles can endure up to 20,000 miles. On the other hand, expensive iridium or platinum spark plugs boast a longer lifespan of around 60,000 miles. Extended-life or long-life spark plugs can last up to 100,000 miles.

2. What factors influence the lifespan of spark plugs?

Ans: The type of spark plug and your vehicle’s make and model play crucial roles in determining their lifespan. Copper plugs tend to have a shorter life, while platinum and iridium plugs generally outlast their counterparts. For optimal performance and when to replace the spark plugs, you need to consult the vehicle’s manual and adhere to the manufacturer-recommended maintenance schedules.

3. How can you identify bad spark plugs and what causes them?

Ans: Common signs of bad spark plugs include rough idling, start-up issues, engine misfires, sluggish acceleration, engine knocking, and declining fuel economy. Causes of the carbon-fouled spark plugs include dirty air filters, fuel injectors, excessive low-speed driving, a rich fuel/air mixture, or prolonged idling.

4. Do bad spark plugs make the car shake while driving?

Ans: Yes, they do. Bad/malfunctioning spark plugs cause uneven fuel burns, fluctuating RPMs, and increased vibrations or noises, resulting in the entire car shaking. This is noticeable even while sitting idle or driving at low speeds.

5. What are the consequences of neglecting spark plug maintenance?

Ans: Ignoring bad spark plugs can lead to engine misfires, causing the vehicle to consume more fuel and work harder for power maintenance. Long-term damage may occur if the issue is not addressed promptly.

6. When should spark plugs be replaced?

Ans: As a general rule, spark plugs should be replaced after about 100,000 miles of driving. You can either change all of these spark plugs at once or one of the badly damaged/broken spark plugs. Adhering to this guideline, or following the manufacturer’s recommended maintenance schedules, ensures consistent engine performance.

7. Is it advisable to drive with faulty spark plugs?

Ans: While technically possible, it is strongly discouraged. Driving with faulty spark plugs can result in reduced engine performance, increased emissions, poor fuel efficiency, and potential damage to other engine components over time. It is always best to replace faulty spark plugs promptly to ensure a smooth and efficient running of the vehicle without any costly repairs or damages. However, any misfire causes the engine to overheat or damage. It is not worth the risk of driving with defective spark plugs. If the car experiencing any of these symptoms, it’s time for a tune-up.

8. Why are iridium spark plugs more expensive?

Ans: Iridium spark plugs are pricier due to the scarcity of iridium, an extremely rare metal mined in specific locations. On average, these spark plugs range from $3 to $20 each, with certain unique models commanding even higher prices.

9. How can you visually identify a bad spark plug?

Ans: A bad spark plug may appear black or covered in sooty deposits if fouled by oil or excessive fuel. A white, blistered appearance indicates overheating and pre-ignition issues. Cracked insulators or worn electrodes are also signs of a problematic spark plug. So, inspecting the spark plugs regularly for these visual cues is crucial to identify and address potential problems.

10. Can you replace a faulty spark plug on your own?

Ans: Yes, you can replace a faulty spark plug on your own with basic automotive knowledge and the right tools. It is a relatively straightforward DIY task for most vehicle owners. Follow the manufacturer’s guidelines, use the correct spark plug type and gap, and take safety precautions. If unsure, seeking professional assistance ensures the job is done correctly.

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